Using older versions of Outlook for Mac for personal email and calendar
For me, it doesn't have to do with OneDrive.
On Windows, new client, if you are attaching a OneDrive file from your recents, it puts it as an attachment but is really a shared link. That works great. Mac has never had the ability. So, OneDrive links just get pasted in line with the body of an email, and they have worked fine.
What wasn't working is when you do an attachment from what the Outlook client believes is a local drive. When you send, you can see it, however, after it's sent, it's not there and it's not stored in the Sent copy either.
For about the last four weeks, it's been fixed and I haven't had the problem.